1. Technical Field
Example embodiments relate to a nonvolatile semiconductor device having a selection transistor.
2. Description of the Related Art
Nonvolatile memory devices may retain data even after power has been discontinued. A representative example of a nonvolatile memory device may include a flash memory. A flash memory may include a cell transistor having a control gate, a floating gate, a source, and a drain. The cell transistor may be programmed or erased according to a F-N tunneling mechanism.
Erasing may be performed on a cell transistor by applying a ground voltage to the control gate of the cell transistor and a voltage greater than a supply voltage to a semiconductor substrate (or a bulk) thereof. Under such an erasing bias condition, a relatively strong electric field may be formed between the floating gate and the bulk due to the relatively large difference between the voltages of the floating gate and the bulk. As a result, electrons present at the floating gate may discharge to the bulk due to an F-N tunneling effect. Consequently, a threshold voltage of the erased cell transistor may decrease.
Programming may be performed on the cell transistor by applying a voltage greater than the supply voltage to the control gate and a ground voltage to the drain and the bulk. Under such a bias condition, electrons may be injected into the floating gate of the cell transistor due to the F-N tunneling effect. As a result, the threshold voltage of the programmed cell transistor may increase. A state in which the electrons are injected into the floating gate may be referred to as a programming state, and a state in which electrons do not exist in the floating gate may be referred to as an erasing state. The threshold voltage of the programmed cell transistor may be greater than about 0 volts, and the threshold voltage of the erased cell transistor may be less than about 0 volts.
Selection transistors may be used to write data to (or read data from) or to apply a predetermined voltage to the cell transistor. The sizes of cell transistors and selection transistors may be reduced to decrease the size of a nonvolatile semiconductor device. However, there may be restrictions in reducing the size of selection transistors due to leakage currents flowing through the selection transistors.